This investigation is directed towards developing an understanding of the molecular basis for ligand specificity in two cholinoreceptive proteins, acetylcholinesterase and the cholinergic receptor in Torpedo electroplax. These studies also involve further characterization of the two proteins with specific emphasis on their structural and functional relationships to the post-synaptic membrane. A variety of ligands with fluorescence probe or quenching properties have been developed which interact specifically and show high affinity for both proteins. Complex formation will be examined by stopped-flow and temperature-jump techniques and this kinetic information will be coupled with conformational studies of the various ligand-protein complexes. We also plan to study the subunit relationships between the various acetylcholinesterase species (lytic and tailed forms), the role played by the tail unit in acetylcholinesterase association with membranes (basement and lipid) and the influence the tail unit may have on orienting the catalytic units on the membrane surface. The fluorescence probes, along with certain bis-quaternary nitroxide compounds (electron-spin resonance probes), will be important labels by which supramolecular structure can be examined in reconstituted systems. Specific studies on the biochemical and conformational characterization of nicotinic receptor desensitization are also planned. Bibliographic references: Taylor, P. Mechanism of Ligand Interactions with Carbonic Anhydrase Studied by Magnetic Resonance Relaxation and Rapid Reaction Methods, J. Pharm. Sci. 64:501-506, (1975). Taylor, P. and Lappi, S. Interaction of Fluorescent Probes with Acetycholinesterase. The Site and Specificity of Propidium Binding. Biochemistry 14:1989-1997 (1975).